1. Field of the Invention
The present invention is directed to a rechargeable battery pack for a power tool, and is more particularly to a battery pack with an over-discharge protection.
2. Description of the Related Art
A rechargeable battery pack has been widely utilized for a power tool. Recently, a lithium-ion battery has been becoming prevalent over conventional nickel-cadmium and nickel-hydrogen batteries because of its high power generating capability and also of a light weight structure. In order to safely utilize the lithium-ion battery for the power tool, it is required to make a protection against over-discharging of the battery, the lithium-ion battery is especially susceptible to over-discharging and may rapidly deteriorate by over-discharging.
In order to avoid the over-discharging, Japanese Patent Publication JP 2005-218174 and JP2005-131770 propose a combination of a battery pack and a power tool.
The battery pack has a battery stack composed of a series connected secondary cells, and incorporates a voltage monitor which is configured to monitor a cell voltage across each one of the secondary cells to provide a detection signal indicative of the cell voltage to the power tool having a motor energized by the battery stack. The power tool includes a controller which controls a discharge current from the battery stack to the motor. The controller is configured to stop feeding the discharge current in an event that the detection signal indicates the over-discharging of the cells. In this connection, the battery pack is required to include a discharge detector which detects and outputs a discharge stop signal to the controller when any one of the secondary cells is over-discharged. Although the discharge detector is only required to operate while the battery pack is connected to the power tool for operating the power tool, it consumes a certain current from the battery stack while it is left disconnected from the power tool because of the necessity of checking the cell voltage of each secondary cell. With this consequence, the battery pack may be exhausted while it is left disconnected from the power tool for a long period of time, thereby eventually deteriorating the secondary cells.